netdev - Re: [RFC PATCH] net: Introduce a socket option to enable picking tx queue based on rx queue.

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Date:   Mon, 11 Sep 2017 23:27:13 -0700
From:   "Samudrala, Sridhar" <sridhar.samudrala@...el.com>
To:     Eric Dumazet <eric.dumazet@...il.com>,
        Tom Herbert <tom@...bertland.com>
Cc:     Alexander Duyck <alexander.h.duyck@...el.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [RFC PATCH] net: Introduce a socket option to enable picking tx
 queue based on rx queue.

On 9/11/2017 8:53 PM, Eric Dumazet wrote:
> On Mon, 2017-09-11 at 20:12 -0700, Tom Herbert wrote:
>
>> Two ints in sock_common for this purpose is quite expensive and the
>> use case for this is limited-- even if a RX->TX queue mapping were
>> introduced to eliminate the queue pair assumption this still won't
>> help if the receive and transmit interfaces are different for the
>> connection. I think we really need to see some very compelling results
>> to be able to justify this.
Will try to collect and post some perf data with symmetric queue 
configuration.

> Yes, this is unreasonable cost.
>
> XPS should really cover the case already.
>   
Eric,

Can you clarify how XPS covers the RX-> TX queue mapping case?
Is it possible to configure XPS to select TX queue based on the RX queue 
of a flow?
IIUC, it is based on the CPU of the thread doing the transmit OR based 
on skb->priority to TC mapping?
It may be possible to get this effect if the the threads are pinned to a 
core, but if the app threads are
freely moving, i am not sure how XPS can be configured to select the TX 
queue based on the RX queue of a flow.

Thanks
Sridhar